ColumnFamilyStore: Propapgate exception cause in sampling wait

Message-Id: <20190311082942.3310-1-calle@scylladb.com>
(cherry picked from commit 5f974bc2bb)

src/main/java/org/apache/cassandra/db/ColumnFamilyStore.java

@@ -136,7 +136,7 @@ public class ColumnFamilyStore extends MetricsMBean implements ColumnFamilyStore
             }
         } catch (InterruptedException | ExecutionException e) {
             futureTableSamperResult = null;
-            throw new RuntimeException("Failed getting table statistics");
+            throw new RuntimeException("Failed getting table statistics", e);
         }
     }